1. Field of the Invention
This invention relates generally to a vehicular motor driving apparatus for driving a vehicle-mounted electric motor and, more particularly, to a vehicular motor driving apparatus in which arrangements are made to prevent a PWM (pulse-width modulation) signal for driving the electric motor from being generated in an AM radio as noise.
2. Description of the Related Art
Conventional vehicular motor driving apparatuses include a vehicle-mounted electric power steering apparatus, a rear wheel steering apparatus for four-wheel steering, an automatic steering apparatus and a power window apparatus. Electric motors of these apparatuses are driven by a given voltage from a battery. In certain conditions, they are PWM-driven to vary a current passing through the motors.
PWM-drive frequencies are preferably set in a range of, for example, 16 kHz to 30 kHz to prevent PWM-drive signals from becoming audible to a driver and to keep the PWM-drive switching loss to a minimum.
Certain conventional vehicular motor driving apparatuses have a PWM-drive frequency fixed, for example, at 17 kHz, and include harmonic components (harmonics) which are an integral multiple of the fundamental frequency (17 kHz) since the PWM-drive has a pulse waveform. When they are positioned closely to a channel frequency of a radio, the harmonics may be received as AM (amplitude modulation) signals by a car radio and be heard as noises.
Assume, for example, that while driving, a driver is listening to a radio broadcast tuned in a channel frequency of 594 KHz. When a PWM-drive frequency is set to be 17 kHz, harmonic components of thirty five (35) times the PWM-drive frequency (17 kHz.times.35=595 kHz) are present closely to the channel frequency of 594 kHz, whereby the deviation (absolute value: 1 kHz) between the channel frequency of 594 kHz and the frequency of 595 kHz of the harmonics arises as an audio signal and can be heard as noise over a radio.
Similarly, when the driver is listening to a radio broadcast of different channel frequency, the harmonic components of a PWM-drive frequency may be heard as noise over the radio as the harmonic components are present closely to the channel frequency.
In order to avoid the phenomenon in which the harmonic components of a PWM-drive frequency are received by a radio and become noises, attempts have been made to delay the rise time and decay time of the pulse waveform of the PWM-drive signal to thereby attenuate the harmonic components. However, delaying of the rise and decay times of the pulse waveform results in an increased switching loss in a motor drive circuit (e.g., a bridge circuit formed of an FET employed as a switching element), whereby the FET produces heat and countermeasures to the heat generation are required. As the countermeasures, one may think of using heat radiation fins, decreasing the ON resistance of FET elements and parallel connecting FETs. However, these countermeasures require the work of careful choosing of parts and increase the number of required parts, thereby increasing the costs of the resulting products.
Consequently, provision is demanded of a vehicular motor driving apparatus which is arranged to prevent the radiation noise associated with the harmonic components of a PWM-drive frequency from being received by a radio receiver, requires a minimum cost of manufacture, and has a high S/N ratio.